The invention relates to an arrangement on a floating device for overhauling offshore hydrocarbon wells, comprising a drill floor for conducting the overhauling, where the drill floor is provided with an opening.
In overhauling hydrocarbon wells, i.e. wells for oil and gas, different tools are employed which are lowered down into the well in a cable or coiled tubing. Occasionally drill strings are also used for overhauling. When overhauling wells which are located below a fixed installation offshore, this overhauling is usually carried out from the fixed installation. When overhauling wells which during their operational phase are not located below any fixed installation, the overhauling is usually carried out from a floating device, which may be a floating drilling platform or a drilling ship.
A well always has a christmas tree with various valves, in which a blow-out preventer forms an integral part. The christmas tree may be located on the seabed, or it may be placed on a platform. A christmas tree with a blow-out preventer which is capable of standing on the seabed can be extremely heavy, and may weigh, for example, 200-250 tons.
An overhaul of an offshore hydrocarbon well with a floating device will often involve an operation where a christmas tree with a blow-out preventer is lowered to the seabed from the floating device. For this reason the floating device""s drill floor may have an opening into which the christmas tree can be inserted for lowering. When the opening is not being used for this purpose, a deck section can be placed in the opening.
When overhauling an offshore well with a floating device according to prior art, a riser is usually placed between the well and the floating device. On account of the motion of the sea, the floating device will normally move vertically in the sea, and in order to avoid damage to the riser it is important that this vertical movement should not be transferred to the riser. For this purpose motion compensators, so-called riser tensioners, are usually employed, which are attached to the floating device and the riser, keeping a uniform tension on the riser independently of the vertical movement of the floating device. The end of the riser, on which an injector head may be placed for insertion of coiled tubing, or another device to which access is required, thereby remains at rest, while the drill floor on the floating device moves in a vertical direction. The maximum vertical movement of the floating device may be 6 meters, and it can therefore be very difficult for personnel located on the drill floor to perform work on devices at the end of the riser.
Norwegian patents No. 123422 and 131352 relates to an offshore construction comprising a vertically movable working deck which is provided with attachment means for a riser which extends from a well to the working deck, where the working deck is kept at an approximately constant level with heave-compensating means.
The object of the invention is to provide an arrangement on a floating device for overhauling offshore hydrocarbon wells, which makes it possible in a practical manner to gain access to the end of the riser which extends from the well to the floating device, even when the floating device is moving vertically relative to the riser.
The object is achieved according to the invention with an arrangement of the type mentioned in the introduction.
Thus the invention relates to an arrangement on a floating device for overhauling offshore hydrocarbon wells, comprising a drill floor for conducting the overhaul, where the drill floor is provided with an opening. A vertically movable working deck which is provided with attachment means for a riser which extends from a well to the working deck can be inserted in the opening. The working deck is suspended in the floating device with riser tensioners in order to maintain an approximately constant tension in the riser during vertical movement of the floating device.
When overhauling a well with a floating device by means of the invention, the riser is attached to the working deck by the attachment means, and the working deck and the riser thereby acquire the same vertical movement relative to the floating device. Personnel who have to carry out work on the end of the riser take their places on the working deck, and the problem of the end of the riser moving relative to the personnel who have to perform the work is thereby solved.
Since it is the relative motion between the riser and the floating device, and not the absolute motion which is the problem in known floating devices, the working deck and the riser are referred to at some places in the following description as vertically movable where this is considered to be most appropriate.
In the invention a known type of floating device is employed, viz. a floating device with an opening in the drill floor. This device may be a floating drilling platform or a drilling ship. As already mentioned, according to prior art an opening of this kind can be used for lowering a christmas tree to the seabed or a deck section.
Riser tensioners will normally be found on a floating device of this type. The working deck can thereby be given a motion-compensated suspension without the need for a comprehensive installation of extra equipment, which is advantageous with regard to space and economy.
Further preferred embodiments, objects and advantages of the invention will be set forth in the detailed part of the description.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.